(1) Field of the Invention
The present invention relates to moving image encoding techniques for encoding a moving image.
(2) Description of the Related Art
Encoding methods including MPEG (Moving Picture Experts Group) method have been drafted as techniques for converting a large amount of moving image information into digital data to write and transfer the digital data. Such encoding methods include MPEG-1 standards, MPEG-2 standards, MPEG-4 standards, and H.264/AVC (Advanced Video Coding) standards and are used as international standard encoding methods.
According to the above-described standards, information about an image whose encoding processing has been completed is used to predict, on a block basis, a target image to be encoded. Then, by encoding the difference in prediction between the target image and an original image, the redundancy of the moving image is eliminated to reduce the encoding amount. In particular, according to the H.264/AVC standards, a drastic improvement in compression ratio is achieved by adopting the intra prediction encoding method that uses pixels surrounding a target block to be encoded.
However, when the intra prediction according to the H.264/AVC standards is executed, a prediction method thereof is too simple to achieve the sufficient prediction accuracy. For example, the intra prediction based on the H.264/AVC standards adopts a unidirectional prediction method in which only one reference pixel is specified so that all pixels in a prediction direction are predicted by use of a pixel value of only one reference pixel as a reference value. Accordingly, there was room for an improvement in prediction accuracy. Therefore, an intra encoding technique, which is capable of improving the accuracy of intra prediction to increase a compression ratio, is required.
As a technique for improving the accuracy in intra prediction, for example, JP-A-2006-352181 discloses that kinds of pixels which can be used for intra prediction are increased by enabling inverting of the whole image before encoding it.
In addition, nonpatent literature 1 discloses a technique in which prediction is performed by use of blocks existing on the upper and lower and right and left sides by changing the order of encoding on a block basis.    Nonpatent literature 1: “Block Based Extra/Interpolating Prediction for Intra Coding” T. Shiodera, A. Tanizawa, T. Chujoh, PCSJ2006, November, 2006.